ATAVISM OR REVERSION. 143 
it does not pass from skin to mucous membrane, never- 
theless, as in the case of the flower, it now and then 
indicates its ancestry by reverting to its original type of 
structure. 
Spurious Atavism.—tIt is so important not to con- 
FIG. 79.—A congenital fold of skin stretching from the thigh 
to the heel of a girl. (After Wolff.) 
found spurious instances with examples of genuine 
reversion or atavism that, even at the risk of being 
tedious, it is necessary to consider briefly a few in- 
teresting cases likely to be misinterpreted on superficial 
examination. 
In 1888 Dr. Julius Wolff, of Berlin, published a 
